Tor and circumvention: Lessons learned. Roger Dingledine The Tor Project

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1 Tor and circumvention: Lessons learned Roger Dingledine The Tor Project 1

2 What is Tor? Online anonymity 1) software, 2) network, 3) protocol Open source, freely available Community of researchers, developers, users, and relay operators Funding from US DoD, Electronic Frontier Foundation, Voice of America, Google, NLnet, Human Rights Watch,... 2

3 The Tor Project, Inc. 501(c)(3) non-profit organization dedicated to the research and development of tools for online anonymity and privacy 3

4 Estimated 300,000 daily Tor users 4

5 Threat model: what can the attacker do? Alice watch Alice! Anonymity network Control part of the network! Bob watch (or be!) Bob! 5

6 Anonymity isn't cryptography: Cryptography just protects contents. Alice Hi, Bob! <gibberish> attacker Hi, Bob! Bob 6

7 Anonymity isn't just wishful thinking... You can't prove it was me! Promise you won't look! Promise you won't remember! Promise you won't tell! I didn't write my name on it! Isn't the Internet already anonymous? 7

8 Anonymity serves different interests for different user groups. Anonymity It's privacy! Private citizens 8

9 Anonymity serves different interests for different user groups. Anonymity Businesses It's network security! It's privacy! Private citizens 9

10 Anonymity serves different interests for different user groups. It's traffic-analysis resistance! Governments Anonymity Businesses It's privacy! Private citizens It's network security! 10

11 Anonymity serves different interests for different user groups. It's traffic-analysis resistance! Human rights activists It's reachability! Governments Anonymity Businesses It's privacy! Private citizens It's network security! 11

12 The simplest designs use a single relay to hide connections. Alice1 E(Bob3, X ) Y Bob1 Alice2 Alice3 E(Bob1, Y ) E(Bob2, Z ) Relay Z X Bob2 Bob3 (example: some commercial proxy providers) 12

13 But a single relay (or eavesdropper!) is a single point of failure. Alice1 Alice2 Alice3 E(Bob3, X ) E(Bob1, Y ) E(Bob2, Z ) Evil Relay Y Z X Bob1 Bob2 Bob3 13

14 ... or a single point of bypass. Alice1 Alice2 Alice3 E(Bob3, X ) E(Bob1, Y ) E(Bob2, Z ) Irrelevant Relay Y Z X Bob1 Bob2 Bob3 Timing analysis bridges all connections through relay An attractive fat target 14

15 So, add multiple relays so that no single one can betray Alice. Alice Bob R1 R3 R4 R2 R5 15

16 A corrupt first hop can tell that Alice is talking, but not to whom. Alice Bob R1 R3 R4 R2 R5 16

17 Alice A corrupt final hop can tell that somebody is talking to Bob, but not who. Bob R1 R3 R4 R2 R5 17

18 Alice makes a session key with R1...And then tunnels to R2...and to R3 Alice Bob R1 R3 Bob2 R4 R2 R5 18

19 Relay versus Discovery There are two pieces to all these proxying schemes: a relay component: building circuits, sending traffic over them, getting the crypto right a discovery component: learning what relays are available 19

20 The basic Tor design uses a simple centralized directory protocol. S1 Trusted directory cache Alice S2 S3 Servers publish self-signed descriptors. Trusted directory cache Authorities publish a consensus list of all descriptors Alice downloads consensus and descriptors from anywhere 20

21 Attackers can block users from connecting to the Tor network By blocking the directory authorities By blocking all the relay IP addresses in the directory By filtering based on Tor's network fingerprint By preventing users from finding the Tor software 21

22 Blocked User Alice Alice Alice Alice Alice R3 Blocked User Alice R4 Bob Blocked User Alice Alice R2 Blocked User Alice Alice Alice R1 Blocked User Alice Alice Alice 22

23 Bridge relays Hundreds of thousands of Tor users, already self-selected for caring about privacy. Rather than signing up as a normal relay, you can sign up as a special bridge relay that isn't listed in any directory. No need to be an exit (so no abuse worries), and you can rate limit if needed Integrated into Vidalia (our GUI) so it's easy to offer a bridge or to use a bridge 23

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29 How do you find a bridge? If you can, go to and it will tell you a few based on time and your IP address Mail bridges@torproject.org from a gmail/yahoo address, and we'll send you a few From your friends and neighbors, like before 29

30 Bridge directory authorities Specialized dir authorities that aggregate and track bridges, but don't provide a public list: You can keep up-to-date about a bridge once you know its key, but can't just grab list of all bridges. Identity key and address for default bridge authorities ship with Tor. Bridges publish via Tor, in case somebody is monitoring the authority's network. 30

31 One working bridge is enough Connect via that bridge to the bridge authority....and to the main Tor network. Remember, all of this happens in the background. How to circumvent for all transactions (and trust the pages you get) is now reduced to How to learn about a working bridge. 31

32 Hiding Tor's network fingerprint We got rid of plaintext HTTP (used by directories). Now clients tunnel their directory requests over the same TLS connection as their other Tor traffic. We've made Tor's TLS handshake look more like Firefox+Apache. When Iran kicked out Smartfilter in early 2009, Tor's old v2 dir design worked again! 32

33 Attacker's goals (1) Restrict the flow of certain kinds of information Embarrassing (rights violations, corruption) Opposing (opposition movements, sites that organize protests) Chill behavior by impression that online activities are monitored 33

34 Attacker's goals (2) Complete blocking is not a goal. It's not even necessary. Similarly, no need to shut down or block every circumvention tool. Just ones that are popular and effective (the ones that work) highly visible (make censors look bad to citizens -- and to bosses) 34

35 Attacker's goals (3) Little reprisal against passive consumers of information. Producers and distributors of information in greater danger. Censors (actually, govts) have economic, political, social incentives not to block the whole Internet. But they don't mind collateral damage. 35

36 Main network attacks Block by IP address / port at firewall Intercept DNS requests and give bogus responses or redirects China: Keywords in TCP packets Iran: DPI to filter SSL when they want Russia: Don't block, just pollute 36

37 What we're up against (1) Govt firewalls used to be stateless. Now they're buying fancier hardware. Burma vs Iran vs China New filtering techniques spread by commercial (American) companies :( How to separate oppressing employees vs oppressing citizens arms race? 37

38 What we're up against (2) Censorship is not uniform even within each country, often due to different ISP policies Attacker can influence other countries and companies to help them censor or track users. We'll see if the GNI (Global Network Initiative) changes that. 38

39 Blocking goes both ways If China blackholes your IP address, you can't reach Chinese websites either. So if exit relays are blackholed, Tor users can't read Chinese websites. :( And if you use dynamic IP addresses, then more and more of your neighbors can't read Chinese websites? 39

40 Only a piece of the puzzle (1) Assume the users aren't attacked by their hardware and software No spyware installed, no cameras watching their screens, etc Users need to know about SSL for gmail. Cookies. Etc. Many people in Iran in June were using plaintext proxies! 40

41 Only a piece of the puzzle (2) Users can fetch a genuine copy of Tor? GPG signatures are great, but nobody knows what that means, and nobody in Burma has my key. Gettor autoresponder. USB key spread by hand. Our secure updater should help. 41

42 Tor gives three anonymity properties #1: A local network attacker can't learn, or influence, your destination. Clearly useful for blocking resistance. #2: No single router can link you to your destination. The attacker can't sign up relays to trace users. #3: The destination, or somebody watching it, can't learn your location. So they can't reveal you; or treat you differently. 42

43 Sustainability Tor has a community of developers and volunteers. Commercial anonymity systems have flopped or constantly need more funding for bandwidth. Our sustainability is rooted in Tor's open design: clear documentation, modularity, and open source. 43

44 Responding to China blocks In late Sept, conflicting advice from experts: Hit 'em in the nose, show that you care about your users Lie low and let it pass. You're about more than China. Tor is a new approach to China bloggers: Find new bridge rather than get software update. 44

45 Publicity attracts attention Many circumvention tools launch with huge media splashes. (The media loves this.) But publicity attracts attention of the censors. We threaten their appearance of control, so they must respond. We can control the pace of the arms race. 45

46 Using Tor in oppressed areas Common assumption: risk from using Tor increases as firewall gets more restrictive. But as firewall gets more restrictive, more ordinary people use Tor too, for more mainstream activities. So the median use becomes more acceptable? 46

47 Other Iran user count Talked to chief security officer of one of the web 2.0 social networking sites: 10% of their Iranian users in June were coming through Tor 90% were coming from proxies in the Amazon cloud 47

48 Trust and reputation See Hal Roberts' blog post about how some circumvention tools sell user data Many of these tools see circumvention and privacy as totally unrelated goals 48

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51 Other ongoing questions How to detect if bridges are blocked (and what to do once we know) Better strategies for giving bridges out (Twitter, better use of social networks; Kaist design project) 51

52 Bridge communities Volunteers run several bridges at once, or coordinate with other volunteers. The goal is that some bridges will be available at any given time. Each community has a bridge authority, to add new bridges to the pool, expire abandoned or blocked bridges, etc. All automated by the Tor client. 52

53 Advocacy and education Unending stream of people (e.g. in DC) who make critical policy decisions without much technical background Worse, there's a high churn rate Need to teach policy makers, business leaders, law enforcement, journalists,... Data retention? Internet driver's license? 53

54 Next steps Technical solutions won't solve the whole censorship problem. After all, firewalls are socially very successful in these countries. But a strong technical solution is still a critical puzzle piece. You should run a bridge! We only have 400. We'd love to help with some trainings, to help users and to make Tor better. 54

Tor: a brief intro. Roger Dingledine The Tor Project

Tor: a brief intro. Roger Dingledine The Tor Project Tor: a brief intro Roger Dingledine The Tor Project https://torproject.org/ 1 What is Tor? Online anonymity 1) software, 2) network, 3) protocol Open source, freely available Community of researchers,